1. Field of the Invention
The present invention relates to a heat-reflection film which effectively reflects heat rays in sunlight while sufficiently passing visible rays therethrough. Such a heat-reflection film is coated on the windshield of a vehicle or the windowpanes of a building so as to reduce the cooling load of the air-conditioning system of the vehicle or the building.
2. Description of the Prior Art
Conventionally, in order to reduce the cooling load of an air-conditioning system of a vehicle or a building, attempts have been made to coat a heat-reflection film on the windshield or the windowpanes so as to intercept the heat rays in sunbeams. This type of film is made of a metal such as gold or silver or a combination of such a metal and a dielectric material. However, this type of film is disadvantageous in that the view therethrough is not clear since visible rays do not sufficiently pass therethrough and in that part of the heat absorbed by the film is transmitted into the interior of the vehicle or the building. On the other hand, in an interference filtering film composed of a plurality of dielectric layers, visible rays can pass therethrough while infrared rays are reflected since the optical thickness of each layer thereof is appropriately selected. A heat-reflection film comprising the conventional interference filtering film comprises one or more dielectric layers of high refractive index material and one or more dielectric layers of low refractive index material. The high refractive index layers and the low refractive index layers are alternately stacked on a base plate which is transparent for visible rays. The optical thickness (the refractive index x the thickness of a layer) of each of the high refractive index layers and the low refractive index layers is .lambda./4, in which .lambda. is the designed wavelength, i.e., the planned wavelength of the heat rays to be reflected by the interference filtering film as a purpose. Each of the high and the low refractive index layers is formed on the base plate by a spattering method, a vacuum evaporation method, a spraying method, or the like so that the formed layer has a predetermined optical thickness. Visible rays penetrate the interference filtering film comprising the above-mentioned dielectric layers having an optical thickness of .lambda./4 while the heat rays are reflected by the interference filtering film. However, the visible rays do not sufficiently pass through the above-mentioned heat-reflection film of the prior art. Part of the visible rays are reflected by the film on the base plate. Therefore, the reflection factor of the base plate coated with the heat-reflection film is a couple of times higher than that of the base plate not coated with a heat-reflection film. Therefore, the heat-reflection film of the prior art does not sufficiently satisfy the requirement that the heat rays be fully reflected while minimizing the reflection factor of the visible rays. When the heat-reflection film of the prior art is coated on a windshield, the view through the windshield is degraded and pedestrians or other drivers are blinded by the light reflected by the windshield.